Greenpeace said earlier that men armed with machine guns had boarded the ship by lowering themselves on ropes from a helicopter.

The environmental activism group said it believed its Dutch-flagged ship was in international waters when the raid began.

It said on Twitter that it lost contact with the ship for around an hour before activists on board managed to get in touch by telephone to say they were locked in.

Russia's foreign ministry earlier Thursday said it had summoned the Dutch ambassador to Moscow Ron van Dartel to issue a protest over the activists' "aggressive and provocative" behaviour.

It said the campaigners who had tried to climb up Gazprom's Prirazlomnaya oil rig had carried out "extremist activity".

Their actions "threatened people's lives and could lead to environmental catastrophe in the Arctic", the ministry said.

Vladimir Chuprov of Greenpeace's Russian office dismissed the foreign ministry claims, saying the ship had not violated any laws.

In a Greenpeace video released Wednesday, coastguard vessels can be seen clashing with activists in small boats, while an official accuses the campaigners of "terrorism."

Greenpeace said that men on the coastguard boats wore balaclavas, fired over 20 shots, and slashed the activists' inflatable boats with knives. Russian coastguards are part of the FSB security service, the successor to the KGB.

The group said its activists would hold a protest Friday morning outside Gazprom's Moscow offices.

Greenpeace says Gazprom – the world's largest gas company – risks causing a catastrophic oil spill in an area with three nature reserves that are home to polar bears, walruses and rare seabirds.

Gazprom is due to start production from the Prirazlomnaya platform in 2014.

Greenpeace argues that the oil platform is "an Arctic disaster waiting to happen".

Director Kumi Naidoo, who led a similar action against the oil rig last year, asked President Vladimir Putin to "restrain the coast guard", saying the campaign group "has done nothing to warrant this level of aggression".

Gazprom has expanded its oil production operations in recent years and describes the oilfield that the Prirazlomnaya will tap into as an essential element of its oil business development strategy.